1. Field of the invention:
The present invention relates to an ink-supplied wire dot matrix printer head having wires supplied with ink at their distal end faces and movable against a sheet of print paper for transferring ink to the sheet in the form of dots to record a character, a figure, a graphic image or the like on the sheet, and more particularly to the construction of an ink tank and an ink guide for guiding ink from the ink tank to the distal end faces of the wires.
2. Description of the Prior Art:
Ink supply systems for a wire dot matrix printer are known in which no ink ribbon is used, but ink is supplied from an ink tank to the distal ends of the wire and transferred from the wires directly to a sheet of print paper. One known ink guide mechanism for such an ink supply system is disclosed in U.S. Pat. No. 4,194,846 and comprises a porous member capable of absorbing ink and for guiding ink from an ink tank with wires contacting the porous member. The porous member contains fine holes with their sizes or diameters varying within a certain range, with the result that the ink absorbing capability varies from porous member to porous member, and excessive and insufficient quantities of ink tend to be supplied to the distal ends of the wire. The quantities of ink retained in the vicinity of the distal ends of the wires widely differ, and the porous member is liable to vary in dimensions or be deformed due to coaction with the sides of the wires. Therefore, the ink densities of formed dots are irregular.
U.S. Pat. No. 4,456,393 discloses another ink supply mechanism in which ink is supplied by a pump from an ink tank to the distal ends of wires. The disclosed ink supply mechanism is disadvantageous in that the construction of a joint between the pump and a printer head is complex and results in an increased cost. It is necessary to provide a sufficient seal so as to gain sufficient pump performance and a large-torque drive source is required for driving the pump. The ink supply mechanism is rendered particularly complex for a multicolor printer head, and such ink supply mechanism is not suitable for use with a small-size printer head.
Another known ink tank construction with an ink-impregnated member 160 such as of a porous material being enclosed in tank 140. The illustrated ink tank construction is of a simple shape and can supply a suitable amount of ink to a printer head body under appropriate capillary attraction by the ink-impregnated member. The ink tank can be impregnated with a large quantity of ink while preventing unwanted ink outflow from an air hole 142 and an ink supply port 141.
When ink is supplied from the ink tank of such a construction, ink in the tank remote from the ink supply port flows toward the ink supply port under a pressure difference developed between ink close to the ink supply port and ink remote therefrom as capillary attraction of the ink-impregnated member in the vicinity of the ink supply port is increased due to ink consumption. However, as can be seen in porous materials, ink-impregnated members are generally subjected to an increased resistance to ink flow and interrupted ink paths preventing a smooth ink flow as the quantity of impregnated ink is reduced. If the ink flow is blocked until a pressure differential sufficient to move ink in the ink tank is produced, then ink remote from the ink supply port remains retained and unused, resulting in a short ink supply duration.
As shown schematically in FIG. 10, the ink tank frequently tends to trap air pockets in the ink-impregnated member. When ambient temperature rises or atmospheric pressure is lowered under such a condition, air communicating directly with the air hole expands and is discharged out of the air hole as indicated by the arrows A without applying any pressure on impregnated ink, whereas the completely trapped air is expanded as indicated by the arrows B while moving the ink surrounding it. When such air pocket reaches the ink supply port, an undesired ink outflow occurs. This causes a smear or ink spot on a sheet of print paper, or ink finds its way.
Accordingly, it is desirable to provide an ink-supplied wire dot matrix printer head which overcomes these problems associated with the prior art.